This invention relates to true ground speed (TGS) sensors of the type employing a Doppler radar system.
It is known that small Continuous Wave (CW) radars may be used to measure true ground speed of vehicles, such as farm tractors; for example see U.S. Pat. No. 3,094,693.
Presently used TGS sensors comprise Doppler systems. An example of such a system comprises a transmitter which may be a semiconductor device. ( e.g., a Gunn diode), suitably placed in an electromagnetic (e.m.) resonant cavity with provisions for applying a D.C. voltage of specified value across the diode. A portion, typically less than 10%, of the D.C. energy is converted to e.m. energy at a frequency determined by the resonant cavity. A means, such as an iris opening, is provided to couple energy from the resonant cavity to a transmission line, typically a section of rectangular waveguide the size of which is appropriate to the e.m. frequency. Typically the transmission line is connected to an antenna which launches the e.m. energy into free space in a directed, shaped pattern. Typically the radar sensor is mounted on a ground vehicle about two feet above the surface of the ground and directed at a slant angle to the surface, either forward or backward-looking.
The e.m. energy striking the surface is both reflected and scattered, and a small portion of it (depending on many factors, including surface material, smoothness and presence or absence of objects, vegetation for example) gets re-directed back to the antenna which focuses it onto the primary feed and into the transmission line where it propagates toward the transmitter. Typically also contained in the transmission line is a receiver comprising a second semiconductor element, commonly known as a detector diode (e.g. a Shottky Barrier diode), placed across the rectangular wave guide and to one side of the center far enough that it intercepts about 1/4 (-6 db) of the outgoing (transmitted) e.m. energy from the transmitter. For the return (received) signal the detector is similarly decoupled, but some compensation for that is obtained by placing it longitudinally (axially) one-fourth guide wavelength from the transmitter output which puts it in the highest e.m. field region of the standing wave. The effect of the absorption of a portion of the transmitted wave is to bias the detector diode into the sensitive region of its characteristic, while the return signal contains the information wanted. A fixed resistor of typically 500 ohms may be connected between the detector diode output and ground, with that output being connected to an amplifier, which completes the receiver.
When there is no relative movement between the vehicle and objects in the antenna beam pattern, the output signal frequency is zero, and the output signal from the detector diode is a DC voltage. This is because the return signal is at the same frequency as the transmitted signal.
If there is a relative motion between the radar sensor and a reflector of e.m. energy in the beam pattern, the received signal will be shifted in frequency either up or down by an amount equal to the Doppler frequency: EQU f.sub.D =2v/.lambda. cos .theta. (1)
where f.sub.D is the Doppler frequency, v is relative radial velocity or velocity along the axis of the radar, .lambda. is the wavelength of the e.m. wave, and .theta. is the angle between the direction of travel of the vehicle and the boresight axis of the radar beam. Whether the received signal frequency is shifted up or down compared to transmitted frequency depends on whether the reflecting surface is moving toward or away from the sensor. The strength of the received signal depends on many factors that are either known or can be estimated.
The choice of frequency for the e.m. energy depends on many factors, including: function, physical size and governmental regulations. Aside from the fact that antenna size for a given value of gain is proportional to wavelength, and hence inversely proportional to e.m. frequency, higher frequencies should be advantageous in magnitude and character of the reflected signal from the surface.
Frequency bands assigned in the U.S.A. by the FCC for non-licensed field-disturbance sensors, Part 15, include 10.525 GHz.+-.25 MHz and 24.125 GHz.+-.50 MHz, the two highest frequency bands assigned. There has been extensive use of the 10.525 GHz band for police radars, and in far larger quantities (hundreds of thousands per year) in microwave intrusion sensors (burglar alarms) and automatic door openers. Consequently, the availability and cost of transceiver (transmitter/detector) assemblies is favorable to the lower frequency. The use of the 24.125 GHz band has thus far been much less in spite of its potential advantages.
A recent interest in use of TGS radar sensors is in connection with the measurement of wheel slip of the driving wheels of a farm tractor pulling heavy loads while working. Depending upon the load and soil conditions, the wheel slip for optimum pull may range from eight to fifteen percent. On the other hand, with the operator of a present-day tractor having a closed cab and being isolated from noise and also quite busy monitoring the vehicle and the attachments and navigating the vehicle, it is quite possible for the wheel slip to considerably exceed optimum values without the operator being aware of it. This leads to non-optimum pulling, a reduction in effiicency, increased tire wear and increased consumption of fuel. The need for a change of gears (speed) or change in load (e.g., plow depth), or both, needs to be anticipated before slippage becomes excessive.
It is known that a measure of wheel slip may be obtained by comparing the speed indicated by a radar sensor mounted on the vehicle to the wheel speed measured by a sensor on the drive wheel, and hence the operator can be alerted to non-optimum wheel slip conditions through the inclusion of means for performing such a measurement and for providing a warning alert to the operation when non-optimum conditions occur.
Another use for TGS information is in connection with operation of seeding equipment to insure that a uniform non-varying amount of seed is applied per unit of distance traveled, or with spraying equipment to insure a uniform distribution of spray with distance. The need for measuring TGS are several, and if the cost can be brought down sufficiently, it is likely that more applications will be found.
The present invention is directed to a new and improved TGS sensor of the type employing a Doppler radar system.
One important objective of the invention is to improve the performance of this type of sensor while concurrently simplifying its design so that a more cost-effective construction results. Hence, one important aspect of the invention involves the construction and arrangement of the microwave assembly which includes the radome, antenna, antenna feed, transmission line, and detector/transmitter assembly. Some of the more specific inventive features relate to the construction and arrangement of the antenna feed and of the antenna reflector, and they provide desirable assembly features, allow a certain amount of beam pattern shaping, and can accommodate optical, including infrared, signals as well as microwave.
One feature involves the use of an ellipsoidal antenna reflector, as opposed to the parabolic reflectors and pyramidal horns which have heretofore been used in TGS sensors. While the gain of a horn antenna may be increased by increasing the length of the horn, an inherent disadvantage is that the volume of space that must be accommodated also increases. A parabolic antenna on the other hand takes the form of a shallower dish. The use of an ellipsoidal antenna reflector similarly reduces the volume of space which the sensor occupies, and it has the advantage over a parabolic reflector in that it allows for better foccussing of the beam. The ellipsoidal antenna reflector illuminates a somewhat smaller area than a parabolic reflector would at a distance of around four feet from the sensor (a typical figure for use in ground vehicles). This distance represents that between the two foci of the ellipse, one being at the antenna feed and the other at the surface of the terrain. In practice the precise distance is not particularly critical.
For plane polarization (as opposed to circular polarization) the beam pattern has two well-defined axes perpendicular to the direction of propagation which is along the boresight axis of the beam. These are referred to as the E-field and the H-field, respectively. The direction of polarization is generally referred to as that of the E-field. Thus vertical polarization signifies that the E-field is perpendicular to the surface. The polarization used affects the amount of signal reflected back from the surface, and vertical polarization is the preferred choice in this application.
To a certain extent the beam width can be controlled independently in the two planes. In the sensor for ground speed measurement it is an advantage to have a beam pattern that is narrower in the vertical plane than in the horizontal, to make the illuminated area on the surface more nearly a circle than for a symmetrical beam pattern since the axis of the sensor is at an angle to the horizontal.
The construction for the preferred embodiment of sensor disclosed herein is one of rugged construction and of more compactness, yet one which functions to achieve an accurate measurement of vehicle movement. The sensor is an enclosed unit in which the radome is at one longitudinal end and an electrical cable connector at the opposite end. The radome is a plastic element and hence it should require no maintenance except for routine cleaning of the plastic window forming the radome. The sensor can also be readily installed and adjusted on a ground vehicle.
A related aspect of the invention involves the use of improved electronic circuitry for providing the output signal which is indicative of vehicle movement.
The foregoing features, advantages and benefits of the invention, along with additional ones, will be seen in the ensuing description and claims which should be considered in conjunction with the accompanying drawings. The drawings disclose a preferred embodiment of the invention according to the best mode contemplated at the present time in carrying out the invention.